One of the most deleterious forms of corrosion of metal structures occurs when these structures are exposed to the electrolytic action of a conductive environment. While not wishing to be bound by any particular theory, it is believed that this "electrochemical corrosion" results from the flow of current from one area of the metal structure (anodic area), through the conductive environment, to another area on the same structure (cathodic area), thereby completing the circuit of a miniature electrolytic cell. At the anodic areas, the metal is oxidized to a positive valence state and leaves the surface in ionic form, ultimately leading to pitting and other forms of gross degradation of the metal. Electrochemical corrosion is generally encountered when such metal structures as tanks and pipelines are buried in the ground or when such structures as ship hulls and off-shore platforms are submerged in sea water. In order to limit corrosive effects under these circumstances, methods for cathodic protection of metal structures have been developed which rely on an external current source or a "sacrificial" anode to impose a negative electrical potential on the metal structure relative to its surroundings. This is believed to effectively turn the whole structure into a cathode, thereby reducing, or eliminating, current flow from the structure to the conductive environment, and thus the corrosion associated therewith.
The above described methods of cathodic protection may be enhanced by coating the metal with an organic film or tape. Thus, for example, in U.S. Pat. No. 4,472,231 to Jenkins, anticorrosion pipewrap system is described wherein a pipe is first coated with a primer based on natural rubber. The coated pipe is then overlaid with an adhesive-coated polyolefin tape, the adhesive comprising a butyl-based rubber, tackifying agent and a cross-linking agent. In addition to providing corrosion protection to the pipes wrapped with the tape, minimal creep of the protective coating composite is obtained in high shear stress environments (i.e., when the pipe is implated inground).
Various silicone compositions have also been utilized as corrosion protective coatings, although not specifically in the area of cathodic protection. In U.S. Pat. No. 4,322,518, assigned to the assignee of the present invention and hereby incorporated by reference, Blizzard discloses silicone coating compositions which comprise a solventless liquid copolymeric organopolysiloxane comprising a curable silicone polymer fluid or gum and a liquid SiO.sub.2 -based resin as the sole curing agent for the fluid or gum. When cured on various substrates, these silicone compositions provide release coatings having controllable release forces for adhesives adhered thereto. A variation on the compositions taught by Blizzard, cited supra, was shown to provide corrosion resistance to ferrous metals which are exposed to moisture and/or salt by Narula et al. in U.S. Pat No. 4,701,380, assigned to the assignee of present invention. In this disclosure, an orgranosilane is added to the above mentioned silicone coating composition in order to improve its adhesion to substrates when cured at room temperature as well as to provide improved corrosion protection.
Blizzard and Swihart, in U.S. Pat. No. 4,537,829, also assigned to the assignee of the present invention and hereby incorporated by reference, teach compositions similar to those described by Blizzard, cited supra. In addition to the curable silicone polymer and liquid SiO.sub.2 -based resin, these compositions further comprise an organosilicon resin consisting essentially of dimethylvinylsiloxy units, trimethylsiloxy units and SiO.sub.4/2 units and a hydrosilylation catalyst. These compositions, when cured, are said to provide improved resistance to fuels and a high degree of corrosion protection to metal substrates.